mx0.authsmtp.com has no CNAME records.
Question and response
Question
dig mx0.authsmtp.com. NS
Answer
The response has no answer records.
Authority
authsmtp.com. 6791 SOA ns0.dmpriest.net.uk. root.dmpriest.com. 2025120302 3600 1800 604800 7200
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0